Counter-bored film-cooling holes and related method

ABSTRACT

A turbine component includes a plurality of film-cooling holes formed in a region of the component to be cooled, the cooling holes having specified diameter, each hole at an exit thereof formed with a counter-bore of predetermined depth; the component having a coating applied thereto at least in the region, wherein the counter-bore provides an area for excess coating material to accumulate without reducing the specified diameter. A method of maintaining cooling efficiency of film-cooling holes in a turbine component where the film-cooling holes have specified diameters and the turbine component has a protective coating therein comprising: a) before coating, forming each film-cooling hole with a counter-bore and an exit end of the film-cooling hole; and b) spraying the coating onto the turbine component at least in areas surrounding the film-cooling holes such that excess coating material accumulates in the counter-bore without reducing the specified diameter of the cooling holes.

BACKGROUND OF THE INVENTION

This invention relates to the configuration of film-cooling holesutilized as part of the cooling circuit in the airfoil portion of aturbine blade or bucket.

Film-cooling has been a major aspect of gas turbine cooling for manyyears. The application of effective film-cooling techniques provides thefirst and best line of defense for hot gas path surfaces against theonslaught of extreme heat fluxes, serving to directly reduce theincident convective heat flux on the surface. As film-cooling holesfirst go into service, they are typically cleaned of all obstructions orunwanted debris. Film holes in this condition may also include certainprotective coatings, either diffusion or thermal barrier coatings (TBC),for such purposes as oxidation protection. In operation, film holes andhot gas path surfaces see a multitude of conditions and environmentswhich can result in the sudden or gradual blockage of holes to variousdegrees, thereby influencing the film-cooling performance to lesser orgreater extents.

It has been discovered, however, that as a TBC coating is sprayed on theairfoil of the bucket, some of the coating material enters the exit ofthe film-cooling holes. Thus, the TBC adheres to the inside surface ofthe film-cooling holes, decreasing the effective exit area of the holesand reducing the film-cooling effect from the design intent.

BRIEF DESCRIPTION OF THE INVENTION

The present invention solves the partial obstruction of film-coolingholes due to TBC's sprayed on the airfoils of the buckets by changingthe configuration of the film-cooling holes to include a counter-bore atthe outlet or exit ends of the film-cooling holes. It is contemplatedthat the counter bores would be applicable to the holes along theleading edge shower head, gill holes and the holes around the bucket tipregion. The counter-bore diameter and depth are specific to the design,and have been optimized for performance. For example, in one exemplaryembodiment, a counter-bore of 0.053 inches on a 0.033 inch diameterthrough-hole extends 0.03 inches from the outlet surface of the airfoilon the minimal dimension.

The general concept of incorporating a counter-bore or flared shape canbe applied to all film-cooling holes for various gas turbine buckets,nozzles and shrouds with TBC application on those parts.

Accordingly, in one aspect, the invention relates to a turbine componenthaving a plurality of film-cooling holes formed in a region of thecomponent to be cooled, the cooling holes having specified diameter,each hole at an exit thereof formed with a counter-bore of predetermineddepth; the component having a coating applied thereto at least in theregion, wherein the counter-bore provides an area for excess coatingmaterial to accumulate without reducing the specified diameter.

In another aspect, the invention relates to a gas turbine bucket havingan airfoil portion and a shank portion, the airfoil portion having aplurality of film-cooling holes therein, each hole at an exit thereofformed with a counter-bore of predetermined depth; the component havinga coating applied thereto at least in the region, wherein thecounter-bore provides an area for excess coating material to accumulatewithout reducing the specified diameter; and wherein the coatingcomprises a first bondcoat layer and a second thermal barrier coatinglayer.

In still another aspect, the invention relates to a method ofmaintaining cooling efficiency of film-cooling holes in a turbinecomponent where the film-cooling holes have specified diameters and theturbine component has a protective coating therein comprising: a) beforecoating, forming each film-cooling hole with a counter-bore and an exitend of the film-cooling hole; and b) spraying the coating onto theturbine component at least in areas surrounding the film-cooling holessuch that excess coating material accumulates in the counter-borewithout reducing the specified diameter of the cooling holes.

The invention will now be described in detail in connection with thedrawings identified below.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a perspective view of a gas turbine bucket with film-coolingholes along the leading edge of the bucket airfoil; and

FIG. 2 is a sketch through the centerline of a film-cooling hole in atest plate showing the build-up of a TBC coating in the hole; and

FIG. 3 is a cross section view of a film-cooling hole on the leadingedge of a bucket in accordance with one embodiment of the invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring now to FIG. 1, there is illustrated a turbine bucket 10constructed in accordance with the present invention including anairfoil 12 mounted on a platform 14. The turbine bucket also includesforward and aft wheel space seals, i.e., angel wings 16, 18,respectively. The buckets 10 are adapted for mounting on the turbinewheel in conventional fashion. The airfoil 12 has a profile including acompound curvature with pressure and suction sides 20, 22, respectively,as well as a leading edge 24 and trailing edge 26.

It is known to apply a thermal barrier coating (TBC) to various regionsof the bucket including adjacent the leading edge 24. Typical TBC'sinclude a first bondcoat layer and a second ceramic coating layer. Thebondcoat layer may be an NiAl-based bondcoat, and the thermal barriercoating layer may be a yttria-stabilized zirconium layer. It is alsoknown to provide film-cooling holes 28 in various regions of the bucketincluding but not limited to the leading edge 24. FIG. 1 illustrates afew representative film-cooling holes 28 for purposes of ease ofunderstanding of the invention.

FIG. 2 is a sketch of a conventionally-shaped film-cooling hole 28formed in a test plate along a simulated leading edge 24. It can be seenthat excess TBC material 30 that coats the simulated leading edge areaaccumulates within a portion of the hole 28, resulting in a decreasedeffective diameter of the hole at the outlet thereof. In a real coolinghole on a bucket, this condition decreases cooling efficiency.

FIG. 3 illustrates in detail one example of a film-cooling hole inaccordance with the invention. Film-cooling hole 32 is shown to belocated at the radially outer end of a bucket 34, along the leading edge36. The film-cooling hole has a nominal diameter “d” that extendsoutwardly from an internal region 38 of the bucket. In accordance withthe invention, film-cooling hole 32 is counter-bored to a diameter “d₁”from its outlet on the leading edge 36 inwardly to a predetermined depth“h.” In a typical example, the nominal diameter d of the film cooinghole 32 is 0.033 in. For this size cooling hole, the counter-bore 40 hasbeen formed with a diameter d₁, of 0.053 in. It will be appreciated thatthe dimensional relationships and dimensions themselves may be varied tosuit different size buckets. It will further be appreciated that theinvention is applicable to cooling holes in any areas on any componentsthat are coated. The counter-bore will provide adequate space toaccommodate excess TBC coating material without reduction of theeffective cooling flow.

While the invention has been described in connection with what ispresently considered to be the most practical and preferred embodiment,it is to be understood that the invention is not to be limited to thedisclosed embodiment, but on the contrary, is intended to cover variousmodifications and equivalent arrangements included within the spirit andscope of the appended claims.

1. A turbine component having a plurality of film-cooling holes formedin a region of the component to be cooled, said cooling holes havingspecified diameter, each hole at an exit thereof formed with acounter-bore of predetermined depth; said component having a coatingapplied thereto at least in said region, wherein the counter-boreprovides an area for excess coating material to accumulate withoutreducing the specified diameter.
 2. The turbine component of claim 1wherein, for a specified diameter of about 0.033 inch, the counter-borehas a diameter of about 0.053 inch.
 3. The turbine component of claim 2wherein the counter-bore has a depth of about 0.030 inch.
 4. The turbinecomponent of claim 2 wherein said coating comprises a first bondcoatlayer and a second thermal barrier coating layer.
 5. The turbinecomponent of claim 4 wherein the bondcoat layer is an NiAl-basedmaterial.
 6. The turbine component of claim 5 wherein the thermalbarrier coating layer is a yttria stabilized zirconium material.
 7. Theturbine component of claim 1 wherein the turbine component comprises agas turbine bucket having an airfoil portion and a shank portion, andwherein said region comprises the airfoil portion.
 8. The turbinecomponent of claim 7 wherein, for a specified diameter of about 0.033inch, the counter-bore has a diameter of about 0.053 inch; and whereinthe counter-bore has a depth of about 0.030 inch.
 9. The turbinecomponent of claim 7 wherein said coating comprises a first bondcoatlayer and a second thermal barrier coating layer; and wherein thebondcoat layer is an NiAl-based material.
 10. A gas turbine buckethaving an airfoil portion and a shank portion, said airfoil portionhaving a plurality of film-cooling holes therein, each hole at an exitthereof formed with a counter-bore of predetermined depth; saidcomponent having a coating applied thereto at least in said region,wherein the counter-bore provides an area for excess coating material toaccumulate without reducing the specified diameter; and wherein saidcoating comprises a first bondcoat layer and a second thermal barriercoating layer.
 11. The gas turbine bucket of claim 10 wherein thebondcoat layer is an NiAl-based material.
 12. The gas turbine bucket ofclaim 11 wherein the thermal barrier coating layer is ayttria-stabilized zirconium layer.
 13. A method of maintaining coolingefficiency of film-cooling holes in a turbine component where thefilm-cooling holes have specified diameters and the turbine componenthas a protective coating therein comprising: a) before coating, formingeach film-cooling hole with a counter-bore and an exit end of thefilm-cooling hole; and b) spraying the coating onto the turbinecomponent at least in areas surrounding the film-cooling holes such thatexcess coating material accumulates in the counter-bore without reducingthe specified diameter of the cooling holes.